The broad objectives of this research are to define systematically a structural and physicochemical framework for describing and predicting the interactions of peptides and proteins with lipid bilayers. Such a framework will help define the fundamental thermodynamic and structural constraints operating in processes such as the insertion and folding of proteins in membranes, peptide mediated membrane fusion, the mode of action of antibiotic peptides such as defensins and magainins, peptide binding to membranes and receptors, the transport of peptide drugs across membranes, and the interaction of signal sequences with membranes. A clear understanding of the physicochemical constraints will be of great value in the design and refinement of biological and biochemical studies of these processes. During the past several years, my laboratory has developed basic methodologies for (a) the use of "host-guest" peptides to determine the incremental thermodynamics of binding of guest amino acids residing in a host polypeptide of fixed composition and for (b) determining the complete structure of fluid bilayers by the joint refinement of neutron and x-ray diffraction data (called liquid-crystallography). By combining these two approaches, one can determine where particular peptides reside in the bilayer, the energetics of their interactions at that location, and how they affect the organization of the bilayer. The research proposed here is designed to develop further liquid-crystallography for bilayer structure determination, to apply it to specific peptide-bilayer structural problems, and to use the 'host-guest' method systematically to define bilayer interaction parameters for each amino acid in several different contexts. The specific goals are: (1) Further develop the joint refinement method using diolcoylphosphatidylycholine and use it to determine the structure of fluid palmitoyloleoylphosphatidylcholine (POPC) bilayers to use as an unperturbed reference standard. (2) Perform a complete structural study of POPC bilayers containing peptides arranged in a transbilayer configuration. The objective is to understand the structure of the helices in the bilayer and their effect on bilayer organization. (3) Use simple pentapeptide host-guest systems to develop bilayer interaction parameters for the individual amino acids in the context of bilayer lipid composition and peptide-sequence neighbor effects. (4) Examine systematically the induction of secondary structure in peptides caused by their binding to the bilayer surface. (5) Continue diffraction studies of oriented lipid multilayers containing the Defensin antibiotic peptide.